1. Field of the Invention
The present invention relates to a method for disguising a microelectronic integrated digital logic circuit which deters attempts to analyze the logical structure of the circuit by reverse engineering.
2. Description of the Related Art
A method for protecting hardware integrated circuits or microcircuits against unauthorized copying and/or use is described in U.S. Pat. No. 4,766,516, entitled "Method and Apparatus for Securing Integrated Circuits from Unauthorized Copying and Use", issued Aug. 23, 1988, to F. Ozdemir et al, and assigned to Hughes Aircraft Company, the assignee of the present invention. According to this technique, at least one additional circuit element that does not contribute towards the function of an integrated circuit (IC), but rather inhibits the proper functioning of the IC in case of an attempted copying, analysis, or other unauthorized use, is fabricated along with the overall IC.
The identities of the additional circuit elements are disguised by forming them with the visible appearance of apparent elements, but with physical modifications that are not readily visible to a copyist but cause them to function in a different manner. Such modifications include very narrow open circuit cuts in metallized connection lines, preferably made with a focused ion beam (FIB) or laser beam; disordering the lattice structure or changing the doping level of a semiconductor region, preferably with a FIB; and injecting electrical charge into a semiconductor region, preferably with an electron beam.
The FIB modifications include implantation of ions into the channel regions of field effect transistors (FETs) to change the threshold voltages thereof and cause them to be always on or always off. The modifications themselves are virtually undetectable. However, it is at least theoretically possible to analyze the actual logical functions of the altered FETs using digital computers programmed to perform sophisticated permutational analysis algorithms.